Used in tuning forks and measuring tapes and other instruments

• Alloy of iron and nickel
• Trademark of an expansion-resistant alloy
• Alloy of nickel and iron
• (trademark) an alloy of iron and nickel having a low coefficient of thermal expansion

Wiktionary Word definitions in Wiktionary
n. An alloy of iron containing 35.5% nickel, and having a very low coefficient of expansion.

Wikipedia Word definitions in Wikipedia
Invar , also known generically as FeNi36 ( 64FeNi in the US), is a nickel – iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name Invar comes from the word invariable , referring to its relative lack of expansion ...